Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle, the information being accessed by means of read/write heads generally located on a pivoting arm which moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor generally located inside the hub or below the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such known in-spindle motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft disposed in the center of the hub. One of the bearings is typically located near the top of the spindle and the other near the bottom. These bearings allow for rotational movement between the shaft and the hub while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional bearing system described above is prone, however, to several shortcomings. First is the problem of vibration generated by the balls rolling on the raceways. Ball bearings used in hard disk drive spindles run under conditions that generally guarantee physical contact between raceways and balls, this in spite of the lubrication layer provided by the bearing oil or grease. Hence, bearing balls running on the generally even and smooth, but microscopically uneven and rough raceways, transmit this surface structure as well as their imperfections in sphericity in the form of vibration to the rotating disk. This vibration results in misalignment between the data tracks and the read/write transducer. This source of vibration limits therefore the data track density and the overall performance of the disc drive system.
Another problem is related to the application of hard disk drives in portable computer equipment and the resulting requirements for shock resistance. Shocks create relative acceleration between the disks and the drive casting which in turn shows up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material and leave permanent deformation and damage on raceways and balls.
Moreover, mechanical bearings are not always scalable to smaller dimensions. This is a significant draw back since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit.
As an alternative to conventional ball bearing spindle systems, researchers have concentrated much of their efforts on developing a hydrodynamic bearing. In these types of systems, lubricating fluid--either gas or liquid--functions as the actual bearing surface between a stationary base or housing and the rotating spindle or rotating hub and the stationary surrounding portion of the motor. For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids, or even air have been utilized for use in hydrodynamic bearing systems. The reason for the popularity of the use of air is the importance of avoiding the outgassing of contaminants into the sealed area of the head disc housing. However, air does not provide the lubricating qualities of oil. Its low viscosity requires smaller bearing gaps and therefore higher tolerance standards to achieve similar dynamic performance.
Thus, in the case of a hydrodynamic bearing employing a liquid lubricant, the lubricating fluid itself must be sealed within the bearing to avoid loss of lubricant which results in reduced bearing load capacity. Otherwise the physical surfaces of the spindle and housing would contact one another, leading to increased wear and eventual failure of the bearing system. Equally seriously, the failure of such a seal or other effort to contain the lubricant within the bearing system would cause the entry of contaminants into the head disc region of the disc drive.
Typically, in the prior art, seals for sealing the fluid within the disc drive utilize a pressurized film on the surface of the liquid air interface. In the case of bearing assemblies which employ ferro-magnetic fluids, the seal is achieved by the means of a magnetic field established at each end of the bearing. However, such seals have not been demonstrated to be reliably effective over a long period of time.
Other obvious shortcomings include the fact that many prior art hydrodynamic bearing assemblies frequently require large or bulky structural elements for supporting the axial and radial loads, as such hydrodynamic bearings do not have the inherent stiffness which results from mechanical bearing assemblies. It is difficult to scale the structural support elements to fit within the smaller disc drive dimensions currently in consumer demands. In other instances, hydrodynamic bearing assemblies suffer from the disadvantages of requiring extremely tight clearances and alignments; this burden makes it difficult to manufacture such assemblies since even a small deviation or aberration can lead to faulty bearings.
Another difficulty with assembly of known hydrodynamic bearing systems is that an essential part of the bearing is the formation of patterns of grooves on one of the two facing surfaces which form the bearing. Such patterns or grooves are quite difficult to form, especially in designs where the grooves run all the way to the edge of a flat surface, or where the entire grooved surface must be an effective part of the bearing. The most common way to form these patterns is stamping or coining. Typically, especially at the edges of the pattern, stress raisers appear during the coining process, which interfere with the establishment of the desired pressure distribution patterns within the bearing.
In some hydrodynamic bearing designs, a thrust plate is utilized which extends perpendicular to the rotating shaft or surfaces. Such thrust plate has in the past frequently required a pattern of grooves on both sides. It is very difficult to stamp or groove a plate on both sides, much more difficult than to stamp the plate only on one side. Such stamping and grooving efforts can again lead to distortions in the thrust plate or stress ridges on the surface of the thrust plate. Since hydrodynamic bearing assemblies suffer from the different disadvantages of requiring extremely tight clearances and alignments, such problems quickly lead to faulty bearings since even a small deviation or aberration in the formation of the groove or stamping of the thrust plate can impose sufficient distortion to overcome the narrow margins or clearances which are allowed for assembly and operation.
Most known hydrodynamic bearing designs are based on a fixed shaft and rotating surrounding sleeve. However, by switching to a rotating shaft, significant improvements in power consumption and vibration response could be achieved with no trade-offs in performance. The power consumption would be decreased by using a smaller diameter shaft which is allowed when the vibration performance becomes less dependent on the shaft stiffness as occurs when the sleeve is stationary and cantilevered or supported from the base.
The vibration performance could also be improved significantly in a rotating shaft design when the angular stiffness of the base-shaft system increases when it is replaced by a base-sleeve system. The sleeve cantilevered from the base has a much higher angular stiffness than the shaft cantilevered from the base as is readily apparent from a study of any of the figures of the present design. Therefore, the adoption of an easily assembled rotating shaft design is highly desirable.